We spend between a quarter and a third of our lives asleep, but that doesn’t make us experts on how much is too much, how little is too little, or how many hours of rest the children need to be sharp at school. Let’s tackle some popular myths about Mr Sandman.
1. You need eight hours of sleep a night
That’s the cliche. Napoleon, for one, didn’t believe it. His prescription went something like this: ‘‘Six hours for a man, seven for a woman and eight for a fool.’’
But Napoleon’s formula wasn’t right, either. The ideal amount of sleep is different for everyone and depends on many factors, including age and genetic make-up.
In the past 10 years, my research team has surveyed sleep behaviour in more than 150,000 people. About 11 per cent slept six hours or less, while only 27 per cent clocked eight hours or more. The majority fell in between. Women tended to sleep longer than men, but only by an average 14 minutes.
Bigger differences are seen when comparing various age groups. Ten-year-olds needed about nine hours of sleep, while adults older than 30, including senior citizens, averaged about seven hours.
We recently identified the first gene associated with sleep duration. If you have one variant of this gene, you need more sleep than if you have another.
Although it’s common to hear warnings about getting too much sleep, it’s not difficult to figure out how much sleep we need. We sometimes overeat, but we generally cannot oversleep. When we wake up unprompted, feeling refreshed, we have slept enough.
In our industrial and urban society, we sleep about two hours less per night than 50 years ago. Like alcohol, this sleep deprivation significantly decreases our work performance and compromises our health and memory.
2. Early to bed and early to rise makes a man healthy, wealthy and wise
Benjamin Franklin’s proverbial praise of early-risers made sense in the second half of the 18th century, when his peers were exposed to much more daylight and to very dark nights. Their body clocks were tightly synchronised to this day-night cycle.
This changed as work gradually moved indoors, performed under the far weaker intensity of artificial light during the day and, if desired, all night long.
The timing of sleep - earlier or later - is controlled by our internal clocks, which determine what researches call our optimal ‘‘sleep window’’. With electric light, our body clocks have shifted later while the work day has essentially remained the same.
We fall asleep according to our (late) body clock, and are awakened early for work by the alarm clock. We therefore suffer from chronic sleep deprivation, which we try to compensate for by sleeping in on free days. Many of us sleep more than an hour longer on weekends than on work days.
My team calls this discrepancy between what our body clocks want and what our social clocks want ‘‘social jet lag’’. This is most obvious in teenagers. Their tendency to sleep longer is biological, not because they are lazy, and it reaches its peak at about age 20.
Studies show that teenagers who sleep later and start school later exhibit improved academic performance, higher motivation, decreased absenteeism and better eating habits.
Yet many cultures reward people who start work early, even if they are operating on reduced sleep. As a result, many successful people are short-sleeping early-risers, such as Margaret Thatcher and Bill Clinton.
Fortunately for those of us who like to hit the snooze button, success is not restricted to early birds. Albert Einstein and Elvis Presley, for example, were late sleepers.
3. Exercise helps you sleep
Exercising may contribute to falling asleep earlier, and it certainly helps us sleep soundly through the night, but it’s light, not physical, activity that proves the German proverb ‘‘Fresh air makes you tired’’.
Exercise often means being outside and getting more light - on average, 1000 times more than indoor levels. Exposure to sunlight synchronises our body clocks with daylight.
Sleep is not only regulated by the body clock, but also by how long we were awake (also known as the build-up of ‘‘sleep pressure’’). But not all waking hours are equal. We will get more tired skiing, for example, than sitting at a desk sending email. This is one reason we sometimes lie awake at the end of a long day at the office despite utter exhaustion.
4. Sleep is just a matter of discipline
Most parents and teachers think that if teenagers are zombies in the morning they just lack the discipline to go to bed early. Although it is true that exposure to computer and television screens late at night makes for late rising, early-to-bed teenagers will still have a hard time getting up at the crack of dawn.
Think of teenagers as early shift workers who suffer the most social jet lag. They go to school at their biological equivalent of midnight, with profound consequences for learning and memory. They suffer from sleep deprivation during the school week and certainly should be allowed to catch up on weekends. However, they should sleep with daylight coming into their bedrooms and should refrain from using light-emitting devices after 10pm.
5. Most couples have very different sleep habits
We have all heard the stories: a woman tries to sleep while her husband is reading; or one spouse needs to sleep in while the other wants to start the day. When I ask lecture audiences whether such scenarios sound familiar, I frequently see a majority of hands go up.
But again this is a matter of biology and genetics, not habits and personal preference. Women generally fall asleep earlier than men, who tend towards night-owlishness.
Women, however, tend to control the sleep times in a partnership. Husbands of women who work late shifts at night, for example, go to bed much earlier when their wives are at home than when their wives are working late, research has found.
One finding that might be surprising, given how much time we spend in our beds, is that men and women don’t seem to give any consideration to sleep patterns when choosing a mate.
Till Roenneberg, a professor of chronobiology and medical psychology at the Ludwig-Maximilians University in Munich, is the author of Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired.